1. Field of the Invention
One or more embodiments of the present invention relate generally to monitoring systems. More specifically, embodiments of the present invention relate to a multi-directional sensor used to monitor flow in hybrid power generation systems for example.
2. Background of the Invention
The U.S. Government has invested in fuel cell technology to advance power generation systems. One significant area of research includes systems in which a fuel cell is combined with another power generation device (a turbine for example) to create a hybrid system that combines the advantages of the two stand-alone systems, resulting in a high fuel-to-electricity conversion efficiency.
Such hybrid systems may be configured in several different ways. An exemplary hybrid system, generally designated 10, is illustrated in FIG. 1. The exemplary system 10 uses one or more high temperature fuel cells 12 (solid oxide or molten carbonate fuel cells for example). A compressor associated with a turbine 16 pressurizes an air stream 18. The pressurized air contacts or otherwise communicates with the fuel cells 12 by means of a recuperator 20, valve 22 and a fuel cell air conduit 24. When fuel 26 (natural gas for example) is provided to the fuel cells 12, the resultant electrochemical reactions in the fuel cells 12 generate electrical energy, subsequently used to power a load 28. It is this supply of pressurized/oxygenated fluid to the fuel cell that increases both fuel cell efficiency and power density of the system 10. In at least one embodiment, the recuperator 20 is illustrated venting to a stock 30. For improved control of the system, some of the air may be routed through conduit 27, bypassing the fuel cell.
High-pressure, high temperature effluent of the fuel cells 12 connects or is otherwise communicated to the turbine 16 an via a post combustor 32 (i.e. an oxidizer) and one or more return loop conduits 25. The pressurized effluent expands in the turbine 16, enabling the compressor 14 to operate, in addition to providing more electrical energy to service the original load 28 or a plurality of loads 28, 34. This use of waste heat or pressurized effluent produces electricity, rather than serving solely as a source of a thermal load. This pressurized effluent utilization enhances the fuel-to-electricity efficiency of the illustrated system 10.
Burners are used as the primary or auxiliary energy source in the turbine portion of the hybrid system. These burners usually have one or more sources or inlets using one or more hydrocarbon based fossil fuels such as, for example, natural gas, liquefied petroleum gas, and liquid hydrocarbon-based fuels. Accurate monitoring and control of such combustion process is very important to ensure the efficient and safe operation of the hybrid systems.
There is a growing need to both measure and control the behavior of flames, the combustion process in the gas turbine combustors and the airflow in the hybrid system. Numerous apparatus, systems and methods are available for measuring flames in burners, and in particular gas turbines. For example, commercially available UV flame detectors may be used to monitor the status (flame on or off) of a flame. Alternatively, a photocell may be used as the detector. However, these types of flame monitoring devices are directed to monitoring the flame and not airflow.
Endoscopes may also be used to visually inspect flames. However, they are generally complicated and expensive pieces of equipment that require careful maintenance. Introduction into high temperature burners or turbines requires external cooling and flushing means. Further, endoscopes are not suitable for monitoring flow direction and velocity in the system.
Differential pressure sensors or transducers have previously been used to monitor flow in systems. However, pressure drop sensors generally only operate in one direction, and are slow to respond to change. Furthermore, differential pressure sensors often require large differentials to provide accurate readings. This pressure drop usually results in a significant drop in total system pressure that is unrecoverable; therefore, a loss in system efficiency occurs.
Alternatively, vane type sensors may be used to monitor flow in the system. Generally, such vane type sensors are even more limited than pressure drop sensors, in that they only monitor flow direction, and not velocity.
Hot wire anemometers use temperature disturbance in the flow to detect velocity and direction of flow. However, hot wire anemometers may be confused by temperature fluctuations, and are therefore considered unreliable for applications with varying temperature, and are limited in their temperature range of operation.
Flame Ionization
Flame ionization detectors (FID) commonly used in gas chromatography use the electrical properties of flames to determine very low concentration of hydrocarbons. Their response has been shown to be proportional to the number of methyl radicals produced in the process of oxidation of a hydrocarbon molecule, and the concentration of the specific hydrocarbon. A fraction of these methyl radicals are chemically ionized through the reaction.CH+O→CHO*→CHO+e-
where CH is the methyl radical and O is atomic oxygen produced in the chemical reactions in the flame. The ionization produced is then detected by the FID.
FID is considered a carbon counting device. The FID response is proportional to the number of carbon atoms or the concentration of hydrocarbons in the sample. Cheng et al., Prog. Energy Combustion Science, vol. 24, 1998, pp. 89-124, describes the equation for the current measured in the FID asi=r(CnHm)Q 
where r is the charge per mole of hydrocarbon, (CnHm) is the molar concentration of the hydrocarbons, and Q is the volumetric flow rate. The linearity of the FID measurements depends on the consistency of charge collection. The Cheng reference is incorporated herein by reference.
Other investigations have shown the feasibility of using flame ionization for monitoring and control of internal combustion (IC) engines. Eriksson et al., Ionization Current Interpretation for Ignition Control in Internal Combustion Engines, L. Eriksson, and L. Nielsen, Control Engineering Practice, Vol. 5 (8), 1997, pp. 1107-1113, demonstrated the feasibility of using in cylinder ionization-current measurements to control IC engine spark advance. Watterfall et al., “Visualizing Combustion Using Electrical Impedance Tomography,” Chemical Engineering Science, vol. 52, Issue 13, July 1997, pp. 2129-2138, demonstrated using impedance topography to visualize combustion in an IC engine.
Commonly assigned U.S. Pat. No. 6,887,069 issued May 3, 2005, and incorporated herein by reference in its entirety, describes a real time combustion-control-and diagnostics sensor (CCADS) which employs flame ionization to monitor combustion processes. CCADS is capable for detecting flow reversal in only one direction. CCADS is not capable of measuring velocity in either the forward or reverse direction.
A need therefore exists in the art for an apparatus and method for measuring flow velocity and direction in a system. The apparatus and method should be capable of operating in high temperature environments, with minimal pressure drop and fast response to directional change.